Surface-active alkyl glycosides are the known reaction products of sugars and relatively long-chain alcohols, more particularly aliphatic primary alcohols preferably containing 8 to 22 carbon atoms. The sugar component may be selected from any of the aldoses or even ketoses in the broad sense (hereinafter referred to as glycoses), including for example glucose, fructose, mannose, galactose, talose, gulose, allose, altrose, idose, arabinose, xylose, lyxose and ribose. Aldoses are preferably used in the synthesis of alkyl glycosides by virtue of their increased reactivity. Among the aldoses, glucose is particularly suitable because it is readily obtainable and available in industrial quantities. Accordingly, among the compounds in question, those preferably used in practice are surface-active alkyl glucosides. Their alkyl radical is preferably derived from primary aliphatic alcohols of synthetic origin, but especially from those of natural origin. The fatty alcohol radical is attached by an acetal bond to a mononuclear glucose residue and/or to oligo or polysaccharide residues. For the purposes of the invention, these names are regarded as synonymous with one another.
Surface-active alkyl glycosides are obtained either by the so-called direct synthesis method from relatively long-chain alcohols or alcohol mixtures and sugars or sugar-yielding polymer compounds, particularly starch, or by transacetalization of alkyl glycoside compounds obtained in a preliminary step with a comparatively short-chain alcohol radical attached by an acetal bond. The acetalization or transacetalization is carried out in the presence of a considerable excess of the relatively long-chain alcohols in the presence of acidic acetalization catalysts. To obtain the alkyl glycosides in pure form from the crude reaction products, the acidic catalysts first have to be neutralized and the excess of the long-chain alcohols removed. The alcohols are generally removed by thin-layer evaporation at considerably elevated temperature, sump temperatures of 160.degree. to 210.degree. C. being quite normal. The reaction product accumulating generally solidifies at temperatures above 100.degree. C. and forms dark-colored masses which are solid at room temperature.
These initial reaction products, which are deep dark brown in color, are unsuitable for the practical application of the alkyl glycosides, for example as highly effective surfactant components for domestic and institutional detergents and cleaning preparations. Accordingly, a regular part of the synthesis process is a post-synthesis step in which the crude reaction products initially obtained are bleached. There are many prior publications relating to the production and recovery of pure surface-active alkyl glycosides in the form of light-colored reaction products, cf. U.S. Pat. Nos. 3,454,690 and 3,839,318, European patent applications 102 558 and 165 721 and, in particular, International patent application WO90/03977 ("A process for the direct production of alkyl glycosides"). According to these documents, oxidative bleaching of the crude alkyl glycoside compounds initially accumulating as dark-colored reaction products is generally necessary as an essential step of the process as a whole. Hydrogen peroxide in particular and/or compounds yielding hydrogen peroxide have proved to be optimal bleaches.
More particularly, the bleaching step is carried out as follows:
On completion of acetalization or transacetalization, the hot reaction mixture is slightly cooled, for example to around 90.degree. C., after which organic or inorganic basic alkali metal, alkaline earth metal and/or aluminium compounds are added in such quantities that not only is the acidic catalyst neutralized, a pH value of at least about 8 and preferably in the range from about 8 to 10 is established. The excess alcohol is then distilled off in a vacuum, more particularly a high vacuum, in standard distillation apparatus, for example in a thin-layer evaporator or in a falling-film evaporator, to residual free alcohol contents of less than about 5% by weight, based on the reaction product as a whole. The distillation residue is then cooled to around 105.degree. C. and an approximately 30 to 60% paste is produced by addition of water. It is this paste-form preparation of the alkyl glycoside compounds which is subjected to bleaching. To this end, an active oxygen compound, preferably hydrogen peroxide, is normally added to the aqueous paste-form preparation with stirring over a period of about 0.1 to 5 hours at approximately 80.degree. C., the pH value being kept in the range from about 8 to 10 during this bleaching step, optionally by addition of alkali, preferably sodium hydroxide. The production process hitherto carried out in practice is characterized by this oxidative bleaching of the aqueous alkyl glycoside paste, in which the water containing partly dissolved alkyl glycosides forms the continuous phase while the insoluble alkyl glycoside excess forms the disperse phase. This known measure leads to light-colored, stable products. However, recovery of the alkyl glycosides as a dry solid or as a constituent of dry active-substance mixtures always requires the removal of appreciable quantities of water, which adds considerably to the cost of the process.